•Current (cd) distributions in PEM fuel cells can reveal maldistribution of gases.•Troublesome gas transport and local flooding are evidenced by cd distributions.•Aged fuel cells exhibit far less ...uniform distributions of current density.
Distributions of current density (cd) in a 100 cm2 single polymer fuel cell have been measured, leading to a 12 × 12 data array. For a freshly matured MEA, the dimensionless distributions have been shown to be of moderate uniformity, whatever the cell current, with average deviations of the dimensionless cd in the range 0.16–0.25 (i.e. cd measured deviated from 16 to 25% from the expected value). More thorough examination of the data acquired showed that the non-uniformity is likely due to maldistribution of the gas in the 23 parallel channels of the multiple serpentine pattern of the commercial bipolar plates. For the sake of comparison, the MEA – including the two GDL was submitted to ageing with a standard cycling protocol (FC_DLC) emulating transportation conditions for 500 h. Characterisation of the cell state of health afterwards revealed visible degradation of the GDL, whereas the catalytic layer and the membrane were little affected. In the aged cell, distributions were shown to be far more non-uniform, with deviations ranging from 0.30 to 0.50: the current distributions were then depending on the cell current density. Presumably, due to the GDL ageing, liquid water produced at high cd, could not be removed efficiently, resulting in very low cd’s near the outlet.
Operation of PEM fuel cells at very low voltages can occur in the case of direct hybridization with supercapacitors, depending on their charge level: in such cases, the cell acts as reliable current ...sources that can be used with low voltage devices. In spite of the growing importance of this electrical mode aging phenomena for such conditions are still unknown. A single 100 cm2 cell was operated for more than 2,000 hours at short circuit. The current delivered by the cell provided with an MEA after suitable maturation, was shown to be approx. 98% of that corresponding to the injected hydrogen flow rate. The state‐of‐health of the cell was followed by impedance measurements and determination of the electrochemical surface area of the cathode and the fuel crossover. Significant, regular decay rate of the cell voltage after the low‐voltage run was observed to range from 30 to 100 µV h−1 depending on the current density. The features of the cell were little‐to‐moderately affected by the particular regime: 43% of ECSA was lost which affects only little the FC performance, and hydrogen crossover remains unchanged. Only the diffusion resistance was increased up to fourfold at high current density.
•Fuel cell directly hybridized with supercapacitor is realistic for FC hybrid vehicle.•Direct hybridization is not detrimental to the FC durability.•Direct hybridization allows to operate with ...reduced minimum gas flow rate.•Reduced minimum gas flow rate is only sustainable in direct hybridization mode.•Direct hybridization allows to decrease hydrogen waste.
Fuel cells (FC) and supercapacitors (SC) directly hybridized allow instantaneous response to peak energy demands in transportation applications. Under urban driving cycle, Fuel Cell Dynamic Load Cycling (FC_DLC), gases are fed with a minimum gas flow rate, corresponding to a safety limit current density (iSL), inducing hydrogen oversupply. To reduce this oversupply, the present work focuses on iSL impact on the FC performance and durability. Two iSL conditions (0.2 and 0.05 A cm−2), were investigated under long-term FC_DLC runs with or without hybridization. At low iSL, hybridization allows 8% reduction in hydrogen oversupply; in addition, non-hybridized FC lifetime was found two-time shorter than that of hybridized FC. Erosion of the microporous layer of the gas diffusion layer (GDL) evidenced by scanning electron microscopy at the cathode, results in poorer water management, in consistence with the sudden voltage drops observed and the high increase in through-plane gas permeability in the GDL.
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In the early 1990s Consumers Energy Co. (CEC) began a fleetwide program to switch to a blend of 30% Western and 70% Eastern coal to reduce the cost of generation. As part of the switch, a non-OEM ...low-NOx firing system was installed on Campbell Unit 1, but it increased fuel line pressure losses. In the late 1990s, CEC hired Alstom Power to develop and implement an emissions-reduction strategy for several of the utility's Alstom-designed boilers, and the one at Campbell Unit 1 was among the first to be upgraded. A key result of the CEC initiative was a set of innovative designs by Alstom engineers that enabled the pulverizers of Campbell Unit 1 to be modified rather than replaced. A summary of the design details and performance results of the pulverizer upgrade is presented. The summary demonstrates that switching to PRB coal can be done without necessitating new pulverizers or major system upgrade.
This paper presents a numerical analysis of the thermal-flow characteristics for a laminar flow inside a rectangular microchannel. The flow of water through channels with thin obstacles mounted on ...opposite walls was analyzed. The studies were conducted with a low Reynolds number (from 20 to 200). Different heights of rectangular obstacles were analyzed to see if geometrical factors influence fluid flow and heat exchange in the microchannel. Despite of the fact that the use of thin obstacles in the microchannels leads to an increase in the pressure drop, the increase in the height of the obstacles favors a significant intensification of heat exchange with the maximum thermal gain factor of 1.9 for the obstacle height coefficient h/H=0.5, which could be acceptable for practical application.
While the ability to crystallize metal nanoclusters has revealed their geometric structure, the lack of a similarly precise measure of their electronic structure has hampered the development of ...synthetic design rules to precisely engineer their electronic properties. We track the evolution of highly‐resolved electronic absorption spectra of gold nanoclusters with precisely mass‐selected chemical composition in a controlled environment. Simple derivatization of the ligands yields larger spectral changes than varying the overall atomic composition of the cluster for two clusters with similar symmetry and size. The nominally metal‐localized HOMO–LUMO transition of these nanoclusters lowers in energy linearly with increasing electron donation from the exterior of the ligand shell for both cluster sizes. Very weak surface interactions, such as binding of He or N2, yield significant state‐dependent shifts, identifying states with significant interfacial character. These observations demonstrate a pathway for deliberate tuning of interfacial chemistry for chemical and technological applications.
Atomically‐precise gold nanoclusters are known for their diverse geometric structures, yet comparatively little has been resolved regarding their electronic structure. Using mass‐selective UV/Vis spectroscopy and ligand derivatization, we show that the cluster electronic structure may be systematically tuned via precise chemical control over the interfacial chemi‐ and physisorptive interactions.
Food is an integral part of everyone’s life. Disposable food serving utensils and tableware are a very convenient solution, especially when the possibility of the use of traditional dishes and ...cutlery is limited (e.g., takeaway meals). As a result, a whole range of products is available on the market: plates, trays, spoons, forks, knives, cups, straws, and more. Both the form of the product (adapted to the distribution and sales system) as well as its ecological aspect (biodegradability and life cycle) should be of interest to producers and consumers, especially considering the clearly growing trend of “eco-awareness”. This is particularly important in the case of single-use products. The aim of the study was to present the current trends regarding disposable utensils intended for contact with food in the context of their biodegradability. This paper has summarized not only conventional polymers but also their modern alternatives gaining the attention of manufacturers and consumers of single-use products (SUPs).
A continually increasing demand for papermaking materials and simultaneously growing disproportion between the request for fiber and the limited resources of wood have forced scientists and the ...papermaking industry to search for the new sources of fibrous raw materials. A new promising set of raw materials for papermaking comes from energy crops. This paper presents Sida hermaphrodita Rusby L., as a non-woody raw material for papermaking. From the studies of chemical composition, it follows that cellulose content of more than 40% characterizes phloem of stems and branches, whereas in xylem exhibits more than 32%. The lowest is the concentration of cellulose in leaves and flowers of Sida. The content of lignin is lower than 24% and 16% in stem xylem and phloem, respectively. In Sida, hemicelluloses and mineral substances stand for being not more than 30% and 2%, respectively. The morphology of Sida cells is similar to hardwood, with fiber length of 0.383, 0.470 and 1.025 mm for parenchyma, xylem, and phloem, respectively. The chemical composition of Sida hermaphrodita together with its morphological characteristics make this raw material suitable for a production of papers intended for printing, writing and tissue.
The aim of this study was to investigate the effects of mini Trailing-Edge Devices (mini TEDs) on the aerodynamic characteristics of the RAE 2822 airfoil. Using ANSYS Fluent software, numerical ...simulations were conducted to analyze the compressible flow around the airfoil at Mach number (Ma) 0.73, considering different angles of attack. The mini TEDs used in the simulation were a Gurney flap and a divergent trailing-edge. The results show that the integration of these mini TEDs at the trailing edge leads to a significant increase in both lift coefficient (CL) and drag coefficient (CD) but, for certain angles of attack, an improvement in the lift-to-drag ratio of the airfoil was obtained. The results showed the potential benefits of using mini TEDs to improve aerodynamic performance in aerospace drone applications.
Emerging evidence suggests that stem cell‐derived extracellular vesicles (EVs) may induce pro‐regenerative effects in ischemic tissues by delivering bioactive molecules, including microRNAs. Recent ...studies have also shown pro‐regenerative benefits of EVs derived from induced pluripotent stem (iPS) cells. However, the underlying mechanisms of EV benefits and the role of their transferred regulatory molecules remain incompletely understood. Accordingly, we investigated the effects of human iPS‐derived EVs (iPS‐EVs) enriched in proangiogenic miR‐126 (iPS‐miR‐126‐EVs) on functional properties of human endothelial cells (ECs) in vitro. We also examined the outcomes following EV injection in a murine model of limb ischemia in vivo. EVs were isolated from conditioned media from cultures of unmodified and genetically modified human iPS cells overexpressing miR‐126. The iPS‐miR‐126‐EVs were enriched in miR‐126 when compared with control iPS‐EVs and effectively transferred miR‐126 along with other miRNAs to recipient ECs improving their functional properties essential for ischemic tissue repair, including proliferation, metabolic activity, cell survival, migration, and angiogenic potential. Injection of iPS‐miR‐126‐EVs in vivo in a murine model of acute limb ischemia promoted angiogenesis, increased perfusion, and enhanced functional recovery. These observations corresponded with elevated expression of genes for several proangiogenic factors in ischemic tissues following iPS‐miR‐126‐EV transplantation. These results indicate that innate pro‐regenerative properties of iPS‐EVs may be further enhanced by altering their molecular composition via controlled genetic modifications. Such iPS‐EVs overexpressing selected microRNAs, including miR‐126, may represent a novel acellular tool for therapy of ischemic tissues in vivo.
This study demonstrates that human induced pluripotent stem cell‐derived extracellular vesicles enriched in proangiogenic miR‐126 (iPS‐miR‐126‐EVs) impacted on several endothelial cell functions essential for ischemic tissue repair including cell survival, migration, proliferation, and angiogenic potential in vitro. Moreover, the iPS‐miR‐126‐EV administration in a murine model of acute limb ischemia increased perfusion and functional recovery of ischemic limb and increased expression of proangiogenic genes in the treated tissues, suggesting their pro‐regenerative activity as a novel acellular tool in ischemic tissue repair. Created with Servier Medical Art (SMART—Servier Medical ART).
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